The present invention relates to a method and an apparatus for producing a high-resolution image.
Solid state imaging devices such as CCDs (Charge Coupled Devices) or C-MOS arrays consisting of imaging surfaces, with pixels arranged in a one or two-dimensional matrix are widely used in digital cameras and scanners.
There is however a growing demand for higher resolution of the images. One way of obtaining a higher resolution is by increasing the number of pixels in the imaging surface of solid state imaging devices. However, if the number of pixels is simply increased in e.g. a CCD to satisfy the need for high resolution, its chip size has to be further increased. On the other hand, in order to increase the number of pixels without changing the present CCD chip size, the integration of pixels must be greatly enhanced. It is technically difficult to prepare such a high-density CCD. Even if development of improved fabrication techniques allows the manufacture of such a CCD in the near future, the circuit configuration for driving such a CCD becomes complicated and power consumption becomes higher, thus entailing new problems, which will result in loss of production efficiency and higher cost.
Other methods for obtaining higher resolution have been used. In scanners the CCD or CMOS array are usually moved in one direction in steps equal to the height of the individual sensors by means of stepper motors, and in special high resolution cameras, the sensor matrix are moved in both an X and an Y directions, to move the individual cells to positions not covered by any cell in the matrix (due to a physical distance between each sensor) or to positions not covered by either a red, green or blue filtered cell. All methods have the limitation that the area of each sensor-cell is limiting the achievable resolution.
In many of these systems, a higher resolution is “simulated” by the use of so-called “sharpening” filters, which digitally amplify high frequency components. These filters are so-called “Finite Impulse Response” (FIR) filters, which only amplify those frequencies limited by the above mentioned sensor-cell area, and thereby only increase those high frequency components, that have already passed the limitations of the system anyway.
U.S. Pat. No. 6,002,810 disclose a method for generating an image having an arbitrary number of pixels starting from a given digitally image having a given number of pixels. This method does not provide a means for improving the resolution and the quality of the image after enlarging will be inferior to the original.
U.S. Pat. No. 4,652,928 relate to a method for improving the actual resolution of an image at the time the image is being established. However, the improvement of the resolution is limited by a factor 2 and is effected solely upon movement in a horizontal direction. Moreover, as already mentioned the achievable resolution is limited by the size of the sensor-cells.
WO 97/12483 discloses an array of CCD cameras, which by means of micro-motion mechanisms moves the array such that a number of images are scanned, thereafter the images are combined to form one high-resolution image. It does not, however, disclose a technical solution to create a high-resolution image from the scanned images.
U.S. Pat. No. 5,754,226 discloses a method to generate a high-resolution image based on a number of displaced low-resolution images. However the accuracy of the method requires that no frequency components above 1/Ph occur to avoid aliasing. To efficiently remove frequencies above 1/Ph a birefringence plate is used which both increases the costs and by removing frequencies above 1/Ph in the low-resolution image also excludes the possibility to regenerate information above this frequency in the high-resolution image.